Regardless of a horse or pony's actual birth date, for most competition purposes a year is added to its age each January 1 of each year in the Northern Hemisphere and each August 1 in the Southern Hemisphere. The exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.
Colt: A male horse under the age of four. A common error is to call any young horse a "colt", when the term actually only refers to young male horses
Filly: A female horse under the age of four.
Foal: A horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling. Most domesticated foals are weaned at five to seven months of age, although foals can be weaned at four months with no adverse physical effects.
Gelding: A castrated male horse of any age.
Mare: A female horse four years old and older.
Stallion: A non-castrated male horse four years old and older. The term "horse" is sometimes used to refer specifically to a stallion.
Yearling: A horse of either Female/Male that is between one and two years old
Different parts of the Horse